This invention relates to dielectric assemblies and, in particular, to dielectric assemblies for electrically isolating manifolds and other components, from a liquid electrolyte fuel cell stack.
In a carbonate (liquid electrolyte) fuel cell stack with external manifolds for gas supply and discharge, the manifolds are electrically isolated from the fuel cell stack by a dielectric member in the form of a picture frame. The dielectric frame must be capable of operating at a voltage difference of between 100 and 1000 volts depending on the number of cells in a stack and the electrical configuration of the stacks when arranged in a power plant. A dielectric frame formed of substantially impermeable Al2O3 (porosity of less than 5% and mean pore size of less than 10 um) has been found capable of providing electrical isolation to thousands of volts.
However, at the fuel cell operating temperature of 650° C., the liquid electrolyte in the fuel cell stack tends to creep over the surface of the dielectric frame. The frame and the stack are only separated by a thin porous gasket (porosity of greater than 70% and mean pore size of greater than 10 um) for gas sealing. This porous gasket becomes filled with electrolyte and as the dielectric frame comes in contact with the liquid electrolyte, the frame becomes wetted.
Once this occurs, a thin continuous layer of conductive liquid electrolyte film forms on the surface of the dielectric. The liquid electrolyte will then flow onto the manifold adjacent the frame. This will occur directly, if the frame and manifold are in direct contact, or through a thin porous gasket (porosity of greater than 70% and mean pore size of greater than 10 um), if such a gasket is used between the frame and manifold. In either case, the electrical isolation provided by the dielectric frame can be compromised and can lead to stack malfunction.
As a result, designers of these frames have looked to develop techniques to prevent or reduce the electrolyte creepage wetting. The aim of these designers is to realize a dielectric assembly able to provide stable long-term dielectric insulation of the liquid electrolyte fuel cell stack from the metallic manifold.
It is, therefore, an object of the present invention to provide a dielectric assembly which overcomes the above disadvantages.
It is a further object of the present invention to provide a dielectric assembly which exhibits increased resistance to dielectric creepage.